Thin film quantum materials

Abstract:

We present a novel surface acoustic wave sensor, incorporating the advantages of multi-frequency and multi-mode operation in a single acoustic device structure. SAW sensors are commonly based on the change of the effective elastic constants or the mass loading of the device due to a different sensor environment. Usually, single acoustic modes at a fixed frequency are employed that are able to detect certain chemical species depending on the coating in the wave's propagation path. Analyzing multiple acoustic modes or multiple excitation frequencies that are each reacting differently on a particular load permits the multi-parameter analysis of gases and liquids. We present a sensor system that is based on floating electrode unidirectional transducers (FEUDTs) allowing for the excitation of a set of 48 equally spaced frequencies and at least two acoustic modes (depending on the material system) in a single transducer structure. FEUDTs are the basis for the efficient excitation of acoustic waves at GHz frequencies without the use of sophisticated nanofabrication techniques. Higher frequencies, on the other hand, are advantageous for the sensitivity of the device. We tested the basic operation of our sensor system by applying it to humidity sensing without a sensitive layer.

Abstract:

We present variable-temperature magnetic force microscopy (VT-MFM) studies of epitaxially grown MnAs layers on GaAs(001). In MnAs, a structural and magnetic phase transition occurs at Tc ≈ 40 °C from the hexagonal, ferromagnetic α-phase below Tc to the orthorhombic, paramagnetic β-phase above Tc. In the investigated MnAs-GaAs system, both phases coexist over a temperature range of ≈ 30 °C below Tc due to the involved strain. Using MFM we are able to distinguish between the ferromagnetic and the paramagnetic phases by measuring topographic and magnetic contrast of the same sample area. For VT-MFM studies, we have employed a temperature stage that allows heating and cooling in a controlled atmosphere with small thermal drift (for this system, the temperature was varied from below 0 °C to above 45 °C). The ratio of the ferromagnetic to the paramagnetic phases shows a temperature hysteresis, i.e. the ratio is dependent on whether the sample was heated or cooled to reach the measurement temperature. Detailed studies of the domains and their arrangements over the hysteretic temperature cycle are shown and compared. Mainly three different domain types are found that are dominant in their respective temperature ranges.

Abstract:

The effect of strain on the local phase transition temperature of MnAs/ GaAs was discussed. It was found that the characteristics temperature T* at which the as-grown films transformed to the paramagnetic β-phase was locally shifted up towards the value of unstrained bulk MnAs. The analysis showed that the film areas exhibiting the higher T* were identified as the region in which the strain in the MnAs film was allowed to relax.

Abstract:

The magnetic out-of-plane component in MnAs/GaAs film was discussed. Its temperature dependence was substantially different from the dominating in-plane magnetization. The analysis showed that the out-of-plane component was due to small isolated magnetic 'grains' within the film.